Infrared (IR) photodetectors have found widespread application in IR thermal imaging systems. Traditionally, the most sensitive IR detectors have been made with mercury-cadmium-telluride (HgCdTe) alloy. However, HgCdTe materials have been refined to the point where they are extremely pure and device performance is now limited by the fundamental properties of the electronic band structure of the HgCdTe crystal, which cannot be improved further. Furthermore, the size of cadmium-zinc-telluride (CdZnTe) substrates, on which the HgCdTe is grown, has been outpaced by other substrate materials, such as GaAs, InP, GaSb, and silicon, which are available in larger sizes. The current CdZnTe substrate size limits the number of HgCdTe infrared detector devices which can be fabricated on a single wafer, which in turn prevents HgCdTe-based infrared detectors from benefitting from the economy of scale seen with larger substrates that can accommodate more devices. Additionally, the wafer size limits the ultimate size of large arrays, which would consume an entire wafer. In the very long IR wavelengths, HgCdTe-based detectors have exhibited problems with (i) compositional variations across the wafer which cause variations of the band gap, (ii) large tunneling dark currents caused by the narrow band gap (<0.1 eV), and (iii) higher defect-related dark currents .
The emergence of thin film growth techniques such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) has allowed the synthesization of a wide range of multiple quantum well (MQW) and superlattice (SL) engineered materials. Such artificial structures are currently of great interest as they afford the possibility of tailoring the electronic structure of the crystal by controlled modification of the crystal: viz., layer thickness, alloy composition, strain, growth orientation, etc. Of particular note, the growth of thin strained layer superlattices (SLSs) using Group III-V materials has opened up new materials for IR detection which can be engineered to be superior to other state-of-the-art materials. Detectors based on such SLSs can be readily manufactured at lower costs compared to HgCdTe, with the same or better performance, owing to their compatibility with advanced III-V device processing technology currently in place to support the diode laser and transistor industries—a commonality and economy-of-scale II-VI-based HgCdTe do not benefit from.
One common SLS detector configuration makes use of binary/binary InAs/GaAs materials. These detectors are known to suffer from an as-yet unidentified defect that reduces carrier lifetime, causes recombination of light-generated carriers before they can be collected and measured at a terminal of the device, and produces excess dark current.
A technique described in W. Shaffer, “IR&D Report on MBE of Bi-containing III-V compounds”, Rockwell Science Center (1982) employed bismuth overpressure with various III-V compounds to achieve reduced background carrier concentration. However, this was not practiced in the context of a superlattice structure.